To explain a logic circuit, write its expression, trace a few input cases, then state the rule in a plain sentence. Naming each gate is only the first step.
This lesson follows reading Boolean expressions and truth tables. It leads into connecting hardware functions to system behaviour.
How do I read the wiring?
Start at the inputs and follow each wire to the first gate. Write what that gate outputs, then feed it into the next gate.
Each gate gives one part of the expression, and the last gate gives the full expression.
Worked example: a door alarm
An original circuit has three inputs: D (door open), S (system armed) and P (panic button). The wiring is:
- An AND gate takes D and S.
- An OR gate takes the output of that AND gate and P.
Expression. Q = (D AND S) OR P.
Trace three cases.
| D | S | P | D AND S | Q |
|---|---|---|---|---|
| 1 | 1 | 0 | 1 | 1 |
| 1 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 0 | 1 |
Explanation. The alarm sounds if the door is open while the system is armed, or if the panic button is pressed at any time.
The mistake that costs marks
One slip is to describe each gate in turn and never state what the circuit does. Another is to change the gate order, which changes the circuit.
| Circuit | Expression | D = 0, S = 0, P = 1 |
|---|---|---|
| OR after AND | (D AND S) OR P | 1, the panic button works alone |
| AND after OR | D AND (S OR P) | 0, the door must be open |
Both circuits use the same three inputs and gates. Only the position of the OR gate differs, and now the panic button works only when the door is open. Always finish with a sentence saying what the circuit does.
Check yourself
A lamp circuit has inputs Day (1 when it is daytime) and Motion. The wiring is: a NOT gate on Day, then an AND gate with Motion. Write the expression, trace Day = 0 with Motion = 1, and explain the rule.
Answer
Expression: L = (NOT Day) AND Motion.
Trace: NOT 0 = 1, then 1 AND 1 = 1, so the lamp is on.
Explanation: the lamp is on only when it is not daytime and motion is detected. In the daytime, NOT Day is 0, so the lamp stays off even with motion.
What to study next
Circuits connect to how hardware behaves. Next, read connecting hardware functions to system behaviour, then try the computing systems and logic practice set.
If you want a teacher to hear your circuit explanations, see online one-to-one Computer Science tuition.